When a compressed air installation which has previously been switched off and vented is switched on again, dangerous and damaging pressure surges can occur which lead to damage of cylinders arranged downstream. In order to prevent this, so-called starting valves for progressive build-up of pressure have been developed.
A prior art starting valve according to is known from EP-B-O 328 472. comprises:
a housing with a first and a second opening, PA1 a channel connecting the two openings. PA1 a piston which is movable to and fro in the housing between a closed and an open position, which in the closed position co-operates with a valve seat and thereby closes the channel, PA1 means for maintaining a low flow of pressure medium--while the piston is in the closed position--between a region with high pressure on one side of the valve seat and a region with pressure building up on the other side of the valve seat, PA1 and a spring element which pushes the piston into the closed position, PA1 wherein the piston has action surfaces which are exposed to the high pressure or to the pressure building up and which generate force components which push the piston into its open or closed position when the difference between the high pressure and the pressure building up falls below a threshold value.
In this known starting valve the two action surfaces are provided in the region of the valve seat on the piston, one action surface being disposed on one side of the valve seat and the other action surface on the other side of the valve seat. The action surface to which the high pressure is applied pushes the piston into its closed position, whilst the action surface to which the pressure building up is applied pushes the piston into the open position. By way of a bypass duct disposed in the piston the high pressure gradually proceeds to the other side of the valve seat where it acts in combination with the second action surface in the sense of opening the piston. Since the second action surface is greater in terms of surface area than the first action surface, after a certain equalisation of pressure the closing force emanating from the first action surface and the spring element is overcome so that the piston moves slowly into the open position.
The end of the piston lying opposite the valve seat is located beyond a seal in ambient air and is guided through the bore in a stopper which closes the device. The diameter of the piston in the region of the seal is smaller in this case than the diameter of the piston in the region of the valve seat.
In many cases it is desirable to provide adjusting means which co-operate with the means for maintaining a low flow of pressure medium in order to alter the flow rate of the low flow of pressure medium.
In the known starting valve these adjusting means are formed by an adjusting screw with which the cross-section of the bypass duct disposed in the piston can be altered. This adjusting screw is disposed in the piston and is adjustable in the longitudinal direction thereof. However, when this adjusting screw is moved, the piston itself must be locked since otherwise it would also turn. An adjustment therefore requires additional costs. In addition the cost of manufacture of this known starting valve is relatively high. Thus the known starting valve only allows one direction of installation into an existing pneumatic plant, since the high pressure may only be applied to the opening which is in communication with the action surface which pushes the piston into the closed position. For the second installation variant, therefore, a starting valve is necessary which not only has another piston but also has a differently constructed housing.
The object of the invention, therefore, is to provide a starting valve which uses a new principle of action facilitating simpler construction or simpler manipulation.